1. Field of the Invention
The invention relates to a temperature-controlled crystal oscillating unit and in particular relates to a temperature-controlled crystal oscillating unit and a crystal oscillator, which can stabilize output frequency, have firmness against shock of falling, and are suitable for miniaturization and mass production.
2. Description of the Related Art
The structure of a conventional temperature-controlled crystal oscillating unit is described below with reference to FIG. 8. FIG. 8 is a schematic diagram illustrating the structure of the conventional temperature-controlled crystal oscillating unit (Non-patent Reference 1). Moreover, FIG. 8 is a planar schematic view of the structure depicted in the cross-sectional view of Non-patent Reference 1.
As shown in FIG. 8, the conventional temperature-controlled crystal oscillating unit comprises crystal electrodes 51a and 51b (excitation electrodes), formed on two sides of an SC (stress compensated)-cut crystal blank 50 that is formed into a planar circle; heaters 52a and 52b that are respectively formed into semicircular shape on two surfaces and along a periphery of the crystal blank 50 for heating the crystal blank 50; and a temperature sensor 53 for detecting a temperature of the crystal blank 50.
The crystal electrode 51a, disposed on a front surface of the crystal blank 50, is connected with an electrode positioned in a top direction of FIG. 8; and the crystal electrode 51b, disposed on a back surface of the crystal blank 50, is connected with an electrode positioned in a bottom direction of FIG. 8. The heaters 52a and 52b are connected with electrodes respectively disposed in a left direction and a right direction of FIG. 8.
The temperature sensor 53 is constructed by a thermocouple, and is in contact with a point on the front surface of a crystal substrate near the heater 52a and on one surface of the crystal blank 50 (the front surface, according to FIG. 8), for detecting temperature. In addition, to prevent heat radiation by thermal conduction, the aforesaid structure is sealed in a vacuum package (TO-8 package), so as to form the temperature-controlled crystal oscillating unit.
A crystal oscillator that utilizes the temperature-controlled crystal oscillating unit comprises a temperature control circuit, which controls the current or voltage to the heaters 52a and 52b based on the temperature detected by the temperature sensor 53. In the case of using an SC-cut crystal blank, the temperature is controlled to be about 85° C.
In addition, “Long term stability (aging) of evacuated hybrid OCXO”, Igor Abramzon et al, 2001 IEEE International Frequency Control Symposium and PDA Exhibition (Non-patent Reference 1) has disclosed technology related to crystal oscillating unit. Non-patent Reference 1 discloses the influence of frequency variation due to temperature and aging, in regard to OCXO.
Furthermore, JP 6-85523 (Patent Reference 1) and JP 11-41032 (Patent Reference 2) have disclosed technology regarding temperature-controlled crystal oscillating unit. Patent Reference 1 discloses a surface elastic wave element, in which a temperature sensor and a heater are disposed on the same surface where the comb-type electrodes of a piezoelectric substrate are formed and at the two sides of the electrodes. Patent Reference 2 discloses a structure which comprises a tube-shaped cap disposed on an outer side of a case of a crystal oscillating unit, and the tube-shaped cap has a temperature sensor and a heater.
In addition, JP 9-153761 (Patent Reference 3), JP 2004-343681 (Patent Reference 4), and JP 2005-124129 (Patent Reference 5) have also disclosed relevant technology. Patent Reference 3 discloses a surface-mount crystal oscillating unit, in which a concave space is formed on a back surface at the bottom of the package, and an electronic component for temperature compensation is disposed in the concave space. Patent Reference 4 discloses a temperature compensated crystal oscillating unit, in which a temperature compensation IC is packaged in a concave on one surface, which is opposite to the surface where an oscillating unit is bonded. Patent Reference 5 discloses a crystal oscillator which uses a thermostat bath, wherein a chip resistor for heat generation, an oscillating element, and a temperature control element having large temperature dependency are disposed on the same surface of a substrate, and directly bonded by a thermo-conductive material.    Patent Reference 1: JP 6-85523    Patent Reference 2: JP 11-41032    Patent Reference 3: JP 9-153761    Patent Reference 4: JP 2004-343681    Patent Reference 5: JP 2005-124129    Non-patent Reference 1: “Long term stability (aging) of evacuated hybrid OCXO”, Igor Abramzon et al, 2001 IEEE International Frequency Control Symposium and PDA Exhibition
However, in the conventional temperature-controlled crystal oscillating unit, the thermocouple is in contact with the crystal blank at one point, and as a result, the temperature cannot be detected accurately and the output frequency thereof is unstable. In addition, the conventional temperature-controlled crystal oscillating unit is damageable to vibration or shock, and the thermocouple may be separated from the crystal by shock of falling, etc. Moreover, the conventional temperature-controlled crystal oscillating unit is formed by performing vacuum-sealing into a package respectively, and thus is difficult to be applied in miniaturization and mass production.